Process for etching aluminum alloy surfaces



Unite States Patent PROCESS FOR ETCHING ALUMINUM ALLOY SURFACES NoDrawing. Application January 31, 1955,

Serial No. 485,303

21 Claims. (Cl. 41-42) This invention relates to the production ofetched aluminum alloy surfaces, and especially concerns novel procedurefor producing etched surfaces of this type hav ing a smoother andsubstantially less nodular finish than is conventionally obtained.

It is well known to etch alkali soluble metals such as aluminum alloyswith a solution having a solvent action on the aluminum alloy surface,such as a hot aqueous alkali solution, e. g., one containing sodiumhydroxide. One method of carrying out the etching treatment is byimmersion of the metal surface to be etched in the alkali solution. Theetched article is then further treated to remove smut, e. g., byimmersion in nitric acid, washed, and dried.

When aluminum alloys are etched in alkali solutions, e. g., containingsodium hydroxide, potassium hydroxide, or equivalent agents as alkali,under some conditions of treatment such as temperature and concentrationof the etching solution, and with some alloys, e. g., zinc and copperalloys of aluminum, a relatively pebbly or nodular etched surface isobtained. This may occur when such alkali etching solutions contain thealkali material as the sole ingredient, or when the solution alsocontains additives such as citric acid or salts thereof, as describedand claimed in application Serial No. 445,197, filed July 22, 1954, byKenneth W. Newman, now Patent No. 2,739,883; or other agents such as,for example, gluconic acid, or salts thereof, sorbitol, and the like. Anetching bath producing a nodular etched surface as aforementioned istermed herein a nodulizing' etching solution. The theory of why thisoccurs is not known to applicants.

However, many applications of etched aluminum alloys 1 require that theetched surface be smoother and less nodular than is obtained under theabove conditions.

One object of this invention is to provide etched surfaces of aluminumalloys having improved appearance.

Another object is to provide procedure for producing etched aluminumalloy surfaces which are smooth and substantially free of bumps andnodules.

Yet another object is the provision of relatively simple and inexpensiveprocedure for obtaining improved etched surfaces on aluminum alloys,particularly copper and zinc alloys of aluminum.

We have discovered that when the etched surface of an aluminum alloytreated in an alkali etching solution is bumpy and nodular, i. e., thesolution is a nodulizing etching solution, if a sulfur-containingmaterial is added to the nodulizing etching solution and a piece of saidalloy is treated in the resulting solution, the etched sur face soobtained is substantially free of bumps or nodules, and is smoother andmore uniform than that produced in the nodulizing etching solution inthe absence of such material.

The sulfur-containing material employed as an additive in the aqueousalkali etching bath according to the invention is of the classconsisting of those having a reactive sulfur and those having a reactiveSH group. These materials include normal sulfides and polysulfides,

Patented June 11, 1957 thio and polythio compounds, and such materialsmay be inorganic or organic in nature. Illustrative examples of thesematerials are elemental sulfur, alkali metal sulfides and polysulfides,such as sodium sulfide and sodium polysulfide, so-ditun thiosulfate,thiuram sulfides and disulfides such as tetramethyl-thiuram sulfide andtetra-methylthiuram disulfide, thioureas, thiono and thio carbamates, e.g., thiourea and alkyl and aryl substituted thioureas such as diethyland diphenyl thiourea, ethyl thio carbamate (CzHsSSCNHz) and ethylthiono carbamate (CzHsOSCNHz), and alkyl and aryl mercaptans such asethyl mercaptan and phenyl mercaptan. The preferred materials are thealkali metal sulfides and polysulfides, sulfur, thioureas, andmercaptans in the order named. These materials may be employedseparately, or mixtures of two or more of these materials can be used.

The etching treatment is carried out in an aqueoussolution containing analkali such as sodium or potassium hydroxide, trisodium phosphate,sodium carbonate or the like, or mixtures thereof. Temperature of thesolution may range say from room temperature, for example, 60 to 70 F.,to about 212 F. Generally, a hot solution is employed, with temperaturemaintained in a range from about F. to about 210 F. during treatment.

The alkali concentration of the etching solution is not critical, andsuch concentration may range say from 0.1 to 10 normal, preferably inthe range of 1 to 3 normal. The amount of sulfur-containing materialadded to the nodulizing etching solution may likewise vary but isusually substantially less than the amount of alkali employed.Generally, from about 0.1 to about 10% of the sulfur-containing materialbased on the weight of the alkali can be used, with amounts in the rangeof about 2 to 6% preferred. The sulfur-containing material in thesolution can be replenished from time to time where the bath is incontinuous use.

In practicing the invention, a test piece of the aluminum alloy to beetched is immersedin the alkali etching solution and etched for a periodsufficient to dissolve a substantial amount of surface metal, e. g., toV inch, under the conditions, e. g., concentration of solution andtemperature, at which a part composed of the same alloy is to be etched.For example, the alkali etchant solution concentration may be 5% or 10%,and the solution temperature may be F. The test piece is then Withdrawn,the smut removed, e. g., by a nitric acid solution, and the samplewashed and dried. If the etched surface so obtained is bumpy or nodular,the solution is a nodulizing etching solution. A sulfur-containingadditive according to the invention is then incorporated in theaforementioned amounts in the nodulizing etching solution. The part tobe etched is then immersed in the resulting solution for the periodrequired to produce the desired depth of etch on the surface of thepart.

The additive employed in the process of our invention, as set forth inthe specification and claims, and which has a reactive sulfur orsulfhydryl group, is one capable of precipitating metallic ions in thealkali solution employed in the etching reaction, such as the ions ofthe metallic alloying constituents of the aluminum alloy processed.

A convenient method of selecting a sulfur-containing material for theprocess of the invention is to choose an alkali solution in the range of0.1 to 10 normal and corresponding preferably to the concentration to beemployed in the etching process in which such material is to be used, e.g., 2 normal. The solution is then heated to the operating temperatureat which etching is to take place, e. g., 190 F., and thesulfur-containing material added to the solution, preferably in anamount in the range of 0.1 to 10% by weight of the alkali, e. g., about3%. A one molal solution of any of the following salts, the metallicelements of which are generally present as alloying 3. v constituents ofthe aluminum alloy to be etched, is then added to the solution: asoluble cuprous, cupric, zinc, or iron salt. If a precipitate is formedwith any one of these salts, which precipitate contains a metal atom anda sulfur atom, for example, the sulfide of the metal, this indicates thesulfur-containing material is suitable for use in the invention process,i. e., it has a reactive sulfur or sulfhydryl group. In practice, if oneof the above salts does not form a precipitate, then another is employeduntil they have all been tried or until at least one of these saltsproduces a precipitate, to show suitability of the particularsulfur-bearing material in our process.

However, if a precipitate is not formed under the above conditions, i.e., at the elevated temperatures of the bath normally employed inaluminum etching operations, this 7 does not necessarily mean that theparticular sulfur-containing material being tested is unsuitable. Underthese circumstances the solution containing such material and one ormore of the above soluble salts is cooled, for example, to roomtemperature to determine whether a precipitate will form at the lowertemperatures. If such precipitate does form, the sulfur-containingmaterial is suitable for use in our process.

By the term precipitate is meant the formation of any observable solidmaterial in the solution, whether in suspension or colloidal dispersionin the solution, or in the form of a relatively dense mass which maydrop to the bottom of the solution which contains a sulfur-bearingmaterial. Since under some circumstances and with certain metals orsalts thereof, e. g., a soluble zinc salt, a precipitate may form whichcontains compounds other than sulfur compounds, it is desirable to testthe precipitate for the presence of such sulfur compounds. If suchsulfur compounds, e. g., zinc sulfide, are present as a solidconstituent of said precipitate then the sulfur-bearing additivematerial is deemed suitable for purposes of the invention, that is, ithas a reactive sulfur or SH- group.

The dark highly adherent film called srnu generally present on thesurface of aluminum alloys following treatment in the alkali etchingsolution may be more easily removed when our sulfur-containing additivesare used according to the invention procedure than is usually the casewhen such additives are not employed. These smuts are insoluble in waterand most acids, and generally require treatment in nitric acid, mixturesof HNOs and H2804, or mixtures of chromic and sulfuric acids for theirremoval. Examples of smut removing compositions which can be used are ato solution of nitric acid, and a solution of the composition disclosedin application Serial No. 448,775, filed August 9, 1954, of Kenneth W.Newman, comprising 77.1% sodium acid sulfate, 1% ammonium bifiuoride,0.1% of a wetting agent, for example, Nacconol Z, which is an alkyl arylsulfonate sold by National Aniline Division, Allied Chemical & DyeCorp., N. Y. C., and 21.8% potassium dichromate. Following such smutremoval, and subsequent washing and drying of the aluminum alloy surfaceof the part etched according to the invention, such surface is found tobe very smooth and uniform, and substantially free of nodules.

When using a nodulizing caustic soda etching solution or a nodulizingetching solution containing an equivalent alkaline etching agent, freehowever of our sulfur-bearing materials, the result is a rough pebblyetched surface as aforementioned. Such surface appears full of smallbumps easily discernible with the naked eye. The bumps are distributedover the surface in a heterogeneous irregular pattern with valleysbetween the bumps. It is believed these bumps are the result of anuneven rate of etch across the entire surface, the etch rate being leastat the apices of the bumps and increasing along the surfaces, movingfrom the top of the bumps into the valleys. The surface is shiny understrong reflected sunlight, and appears visually as illuminated acrossthe surface with no interference or refraction phenomenon since no coloris discernible.

However, if as little as 1% of the sulfur-containing material of theinvention is added to the nodulizing etching solution, the etchedsurface obtained is smooth and while a surface etch pattern is visiblein the form of barely discernible striations, no discrete hillocks areobservable even with a magnifying 2 power lens. The surface ap pearscrystalline but smooth under strong reflective sunlight and iridescentand colored, indicating either interference or refraction phenomenon.This surface may be termed jewel-like for this reason.

The ditference in surface, i. e., bumpy, large grained and irregular inthe case of the nodulizing alkali etchant solution free ofsulfur-containing material, and the smooth, fine grained, non-bumpysurface when employing such additive material in said solution isreadily discernible in any form of illumination. or even by feel. Theetched surface obtained when using the nodulizing etchant solution freeof the sulfur-bearing materialof the invention will be termed irregularor coarse grained, as well as nodular, and when such sulfur-containingmaterial is used in said solution, smooth or fine grained.

It is to be understood that comparisons of the coarse grained nodularsurface obtained on an aluminum alloy under nodulizing conditions, withthe substantially nodulefree fine grained surface produced by use of oursulfurbearing materials, e. g., sodium sulfide, according to theinvention, is based on the use of the same etching conditions, i. e.,the same alkali etchant such as caustic soda, the same solutionconcentration, and the same temperature, and also on the use of the samealuminum alloy, except that the etching solution in one case wherein thesmooth fine grained surface is obtained also contains the sulfur bearingmaterial. Thus, for example, the etched surfaces of two aluminum alloyparts of the same alloy composition can be compared for the inventionpurposes by etching each for the same period of time in the same 5%caustic soda etching solution and at the same temperature of F. exceptthat in one case the solution contains our sulfur-bearing material.

Examples of aluminum alloys which can be treated to from a smooth etchedsurface according to the invention are as follows:

Cu, 0.30% Mu, 2.1-2.9%Mg, 0.18-0.40% Cr, 5.1-e.1% Zn, 0.20% Ti, Rem. Al.

Other aluminum alloys may also be etched employing the inventionprinciples, to obtain the invention results.

The following are examples of practice of the invention, all quantities,where not otherwise specified, being in terms of parts by weight.

Example 1 An aqueous etching solution was prepared by dissolving anetchant composition in water to produce a solution concentration of 10%.The etchant composition was composed of 96 parts of caustic soda and 4parts of sodium sulfide. An article having a 248 aluminum alloy surfacewas etched in this solution while maintained at a temperature of about190 F. The article was then withdrawn from the bath, the smut removedfrom the etched surface of the article by treatment with a solution ofthe smut removing composition of application Serial No. 448,775,described above, and the article washed and dried. The etched aluminumalloy surface of the article was fine grained, smooth, and substantiallyfree of nodules. v

spam-49o .5 Example 2 An etching solution was prepared composed of about8% by weight of a mixture of NaOH and thiourea (2 parts thiourea to 98parts NaOH). A 24S aluminum alloy was immersed in the solution andetched :therein while the solution was maintained at a temperature ofabout 180190 F. The etched surface of .the alloy obtained after smutremoval, washing, and drying was fine grained, smooth, and substantiallynodule-free.

Example 4 Results similar to Example 1 were obtained .by employing theprocedure and conditions set forth therein, but substituting for theetchant composition of Example 1 an .etchant composition composed of 95parts of caustic soda and 5 parts of phenyl mercaptan.

Example 5 Results along the lines of Example 1 may be obtained employingthe procedure and conditions of Example ,1, but replacing the sodiumsulfide with tetramethyl thiuram sulfide.

Example 6 A test piece of 24S aluminumalloy was-immersed in a 5% causticsoda etching solution and etched therein at a solution temperature of190 F. until the exposed surface was etched to a depth of about /a inch.The article was then withdrawn from the bath, the smut removed from theetched surface by treatment with a solution of the smut removingcomposition of application Serial No. 448,775, described above, and thearticle washed and dried. The etched surface so obtained was observed tohe coarse grained andnodular, and composed of bumps or hillocks ofrelatively large size. This indicated the above solution was anodulizing etching solution.

Another aqueous etching solution was prepared by dissolving an etchantcomposition in water to produce a solution concentration of 5%. Theetchant composition was composed of 97 parts of caustic soda and 3 partsof sodium sulfide. An article having a 248 aluminum alloy surface wasetched in this solution while maintained at a temperature of 190 F. Thearticle was then withdrawn from the bath, the smut removed from theetched surface by treatment with a solution of the smut removingcomposition of application Serial No. 448,775, described above, and thearticle washed and dried. The etched aluminum alloy surface so obtainedwas fine grained and in the form of striations with no discerniblenodules or hillocks.

Example 7 The same procedure was carried out as in Example 6, exceptthat the etchant composition of the second etching solution was composedof 99 parts of NaOH and 1 part of sodium sulfide. those obtained inExample 6.

While we have described a particular embodiment of our invention for thepurpose of illustration, it should be understood that variousmodifications and adaptations thereof may be made within the spirit ofthe invention as set forth in the appended claims.

We claim:

1. A method of etching an article having an aluminum The resultsobtained were similar to .alloy surface to produce asmooth substantiallynodulefree etched surface, which comprises treating said alloy surfacewith an alkali etching solution having dissolved 'therein asulfur-containing material of the class consisting of those having areactive sulfur and those having a reactive SH- group, saidsulfur-containing material being capableof forming a precipitate with anion taken from the group consisting of copper, zinc and-iron ions when asolution-containing such ion is added to an alkali solution of saidsulfur-containing material.

2. A methodof etching an article having an aluminum alloy surface .toproduce a smooth substantially nodule- -free etched surface, whichcomprises treating said alloy surface with a hot alkali etching solutioncontaining caustic soda and having dissolved therein a sulfur-containingmaterial of the class consisting of those having a reactive sulfur andthose having a reactive SH.- group, said sulfur-containing materialIbeing capable of forming aprecipitate with an iontaken from the groupconsisting of copper, zinc and iron ions when a solutioncontaining suchion is added 'tofan alkali solution of said sulfur-con- .tainingmaterial.

'3. A method of etching an article having an aluminum alloy surface toproduce a smooth substantially nodulefree etched surface, whichcomprises treating said alloy surface with a hot alkali etching solutionhaving dissolved therein a sulfur-containing material of the classconsisting of those having a reactive sulfur and those having a reactiveSH group, said sulfur-containing material being capable of .forming aprecipitate with an ion taken from the group consisting of copper, zincand iron ions when a solution containing suchion is added to an alkalisolution of said sulfur-containing material, said sulfur-containingmaterial being present in an amount of from about 0.1% to about 10% byweight of said alkali.

4. A methodof etching an article having an aluminum alloy surface toproduce. a smooth substantially nodulefre'e etched surface, whichcomprises treating said alloy surface with an alkali etching solutionhaving dissolved therein a mercaptan.

-5. A method of etching an article having an aluminum 'alloysurface toproduce a smooth substantially nodule- :free etched surface, whichcomprises treating said alloy surface with a hot alkali etching solutioncontaining causticsodaand having dissolved therein from about 0.1toabout 10% by weightofsaid caustic soda, of a mercaptan.

6. A method as defined in claim 5, wherein said mercaptan is phenylmercaptan.

7. A method of etching an article having an aluminum alloy surface toproduce a smooth substantially nodulefree etched surface, whichcomprises treating said alloy surface with a hot alkali etching solutionhaving dissolved therein a thiourea.

8. A method of etching an article having an aluminum alloy surface toproduce a smooth substantially nodulefree etched surface, whichcomprises treating said alloy surface with a hot alkali etching solutioncontaining caustic soda and having dissolved therein from about 0.1 toabout 10% by Weight of said caustic soda, of thiourea.

9. A method of etching an article having an aluminum alloy surface toproduce a smooth substantially nodulefree etched surface, whichcomprises treating said alloy surface with a hot alkali etching solutionhaving dissolved therein a thiuram sulfide. g

10. A method of etching an article having an aluminum alloy surface toproduce a smooth substantially nodulefree etched surface, whichcomprises treating said alloy surface with a hot alkali etching solutionhaving dissolved therein a thiuram disulfide.

11. A method of etching an article having an aluminum alloy surface toproduce a smooth substantially nodulefree etched surface, whichcomprises treating said alloy surface with a hot alkali etching solutioncontaining caustic soda and having dissolved therein tetramethyl thiuramsulfide.

' 12. A method of etching an article having an aluminum alloy surface toproduce a smooth substantially nodulefree etched surface, whichcomprises treating said alloy surface with a hot alkali etching solutioncontaining caustic soda and having dissolved therein tetramethyl thiuramdisulfide.

13. A method of etching an article having an aluminum alloy surface toproduce a smooth substantially nodulefree etched surface, whichcomprises treating said alloy surface with an alkali etching solutionhaving dissolved therein a sulfur-containing material having a reactivesulfur, said sulfur-containing material being capable of forming aprecipitate with an ion taken from the group consisting of copper, zincand iron ions when a solution containing such ion is added to an alkalisolution of said sulfur-containing material.

14. A method of etching an article having an aluminum alloy surface toproduce a smoothsubstantially nodulefree etched surface, which comprisestreating said alloy surface with an alkali etching solution havingdissolved therein a sulfur-containing material having a reactive SH-group, said sulfur-containing material being capable of forming aprecipitate with an ion taken from the group consisting of copper, zincand iron ions when a solution containing such ion is added to an alkalisolution of said sulfur-containing material.

15. A method of etching an article having an aluminum alloy surface,which comprises adding a sulfur-containing material of the classconsisting of those having a reactive sulfur and those having a reactiveSI-I- group, to a nodulizing alkali etching solution, saidsulfur-containing material being capable of forming a precipitate withan ion taken from the group consisting of copper, zinc' and iron ionswhen a solution containing such ion is added to an alkali solution ofsaid sulfur-containing material, and treating said alloy surface in theresulting solution to obtain a smooth substantially nodule-free etchedsurface.

16. A method of etching an article having an aluminum alloy surface,which comprises adding a sulfur-containing material of the classconsisting of those having a reactive sulfur and those having a reactiveSH- group, to a nodulizing hot alkali etching solution containingcaustic soda, said sulfur-containing material being capable of forming aprecipitate with an ion taken from the group consisting of copper, zincand iron ions when a solution containing suchion is addedlto an alkalisolution of said 17. A method of etching an article having an aluminumalloy surface, which comprises adding a thiourea to a nodulizing hotalkali etching solution containing caustic soda, and treating said alloysurface in the resulting solution to obtain a smooth substantiallynodule-free etched surface.

18. A method of etching an article having an aluminum alloy surface,which comprises adding a mercaptan to a nodulizing hot alkali etchingsolution containing caustic soda, and treating said alloy surface in theresulting solution to obtain a smoothsubstantially nodule-free etchedsurface.

19. A method of etching an article having an aluminum alloy surface,which comprises adding a thiuram sulfide compound to a nodulizing hotalkali etching solution containing caustic soda, and treating said alloysurface in the resulting solution to obtain a smooth substantiallynodulefree etched surface.

20. A method of etching an article'having an aluminum alloy surface toproduce a smooth substantially nodulefree etched surface, whichcomprises treating said alloy surface with an alkali etching solutionhaving dissolved therein a thio carbamate.

21. A method of etching an article having an aluminum alloy surface toproduce-a smooth substantially nodulefree etched surface, whichcomprises treating said alloy surface with an alkali etching solutionhaving dissolved therein a thiono carbamate.

References Cited in the file of this patent UNITED STATES PATENTS1,639,070 Allen Aug. 16, 192.7 1,785,513 Calcott et a1. Dec. 16, 19302,461,228 Miles Feb. 8, 1949 2,628,199 Lowenheirn Feb. 10, 19532,650,875 Dvorkovitz Sept. 1, 1953 2,692,188 Chester Oct. 18, 1954FOREIGN PATENTS 342,601 Great Britain Feb. 5, 1931

1. A METHOD OF ETCHING AN ARTICLE HAVING AN ALUMINUM ALLOY SURFACE TOPRODUCE A SMOOTH SUBSTANTIALLY NODULEFREE ETCHED SURFACE, WHICHCOMPRISES TREATING SAID ALLOY SURFACE WITH AN ALKALI ETCHING SOLUTIONHAVING DISSOLVED THEREIN A SULFUR-CONTAINING MATERIAL OF THE CLASSCONSISTING OF THOSE HAVING A REACTIVE SULFUR AND THOSE HAVING A REACTIVESH-GROUP, SAID SULFUR-CONTAINING MATERIAL BEING CAPABLE OF FORMING APRECIPITATE WITH AN ION TAKEN FROM THE GROUP CONSISTING OF COPPER, ZINCAND IRON IONS WHEN A SOLUTION CONTAINING SUCH ION IS ADDED TO AN ALKALISOLUTION OF SAID SULFUR-CONTAINING MATERIAL.